Stabilization of jet fuel

ABSTRACT

The stability of distillate type jet fuels is improved by cathodic hydrogenation in an electrolytic cell with a proton permeable membrane separating cathode and anode compartments; a source of hydrogen is oxidized in the anode compartment to form protons which permeate the membrane to effect a cathodic reduction of the nitrogenous components of the fuel in the cathode compartment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 62/042,363 filed Aug. 27, 2014, herein incorporated by reference inits entirety.

FIELD OF THE INVENTION

This invention relates to a method for stabilization of distillate typejet fuel.

BACKGROUND OF THE INVENTION

The stability of distillate type jet fuel, either during storage orduring use, has always been a significant technical challenge and isbecoming more of an issue recently with increasing volumes of crackedstock entering the jet pool. Nitrogen compounds are known to bedeleterious to the stability of hydrocarbon fuels by promoting theformation of highly intractable sediment or sludge under storageconditions and during thermal stress. Frankenfeld et al, demonstratedthat “the rate of sediment formation was dependent on the presence ofnitrogen compounds . . . The initial reaction rate was approximatelyfirst order in nitrogen concentration . . . . and appears to involve afree-radical oxidative self-condensation of the nitrogen compound.”(Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 608-614). This is still theconsensus view of the issue today, although the increased use of crackedstocks in the jet pool has added olefinic materials as anothercontributor to jet fuel instability. Therefore, nitrogen removal is akey element of improving the stability of kerosene jet fuels.

The conventional and standard approach to stabilizing jet fuel is to usecatalytic hydrodenitrogenation to achieve this goal. Catalytic processesrequire ever more severe conditions (equipment, temperatures, pressure,residence time, hydrogen consumption) to achieve ever lower levels ofnitrogen in the final product. More severe conditions generallytranslates into higher processing costs and a more costly product.

Alternative desulfurization processes have received considerableattention, including electrochemical process such as those reviewed byLam in Fuel Processing Technology 98 (2012), 30-38. Among thosetechniques are the variant electrochemical processes in which hydrogengenerated in an electrolytic cell is used to reduce the sulfur compoundsto inorganic form (H₂S) which can then be removed from the hydrocarbons.Examples of such desulfurization methods are described by Greaney inUS2007/0108101; US 2009/0159427; 2009/0159500; US2009/0159501 and US2009/0159503. Exploitation of the intrinsic electrical conductivity atelevated temperatures in the range of about 200 to 400° C. in theelectrochemical desulfurization methods described by Greaney dispenseswith the use of liquid electrolytes but does require the use of the hightemperatures which restrict the material choices in the processingequipment.

While conventional hydrotreating processes effect denitrogenation alongwith desulfurization little attention has been given to the problem ofimproving jet fuel stability by the removal of the nitrogenouscompounds.

SUMMARY OF THE INVENTION

An electrochemical approach can be used to improve the stability ofdistillate type jet fuels by denitrogenation under mild conditions (e.g.60° C., atmospheric pressure) utilizing a polymer electrolyte membranefuel cell, hydrogen gas or water and electricity. There are severaladvantages to this approach. First, this process can achieve equivalentresults at milder conditions and could be used as a “polishing” stepafter standard hydrotreating. This could thereby reduce the severityrequired in the hydrogenative step. This could reduce process costs forexample, by reducing hydrogen consumption and extending catalyst life byoperation at lower temperatures. Fuel cells could also be distributed tofuel storage facilities and used locally to stabilize fuel, once it isdelivered and ensure that it remains stable over time. These fuel cellsare commercially available and are designed for ease of installation fordistributed power generation.

According to the present invention, the distillate jet fuel isdenitrogenated by cathodically hydrogenating the fuel in a electrolyticcell analogous to a proton exchange membrane (PEM) fuel cell in which aproton permeable membrane separates the anode and cathode compartments.A source of hydrogen is oxidized to form protons in the anodecompartment which permeate the membrane to effect a cathodic reductionof the nitrogenous components of the fuel in the cathode compartment.

The electrolytic cell in which the denitrogenation is carried out is adivided cell in which a proton permeable membrane separates the anodeand cathode compartments. A source of hydrogen is oxidized in the anodecompartment to form protons which permeate through the membrane to thecathode side through which the jet fuel is passed. The electrolyticdenitrogenation is carried out by cathodic reduction of the nitrogenspecies to ammonia which takes place in the cathode compartment of thecell. The ammonia is released from the treated fuel in a liquid/gasseparator with separation being enhanced by stripping with inert gas orby increase of temperature.

DRAWINGS

The single FIGURE of the accompanying drawings is a simplified sectionaldiagram of a divided cell electrolysis cell useful for jet fueldenitrogenation.

DETAILED DESCRIPTION Jet Fuel

The present invention is applicable to the treatment of distillate(kerosene type) jet fuels such as Jet-A, Jet A-1, JP-5 and JP-8. Thesefuels typically have an initial boiling point of not less than 150° C.and an endpoint not more than 300° C. (ASTM D86). Flashpoint is not lessthan 38° C. (ASTM D56 or D3828) and freeze point not more than −40° C.(−47° C. for Jet A-1 and JP-8). Smoke point (ASTM D1322) not less than25 mm of smoke point or not less than 18 mm if naphthalenes are not morethan 3 vol. pct (ASTM D1840). The sulfur specification is a maximum of0.3 wt. pct; nitrogen is not specified but, as noted above, is importantto stability. Thermal stability (JFTOT, ASTM D3241) is defined as apressure drop less than 25 mm Hg and deposits less than 3.

Denitrogenation

The major nitrogen component of distillates, such as jet fuels, aremolecules which are difficult to remove by conventional hydroprocessingmethods without using severe conditions, such as high temperatures andhydrogen pressures, these molecules are however, converted by thepractice of the present invention to ammonia which can be readilyremoved. Nitrogen species such as alkylbenzo derivatives of pyridine andpyrrole.are removed together with mixed heterocyclic species containingone nitrogen (pyridinic, quinolinic) with one sulfur (thiophenic) oroxygen (hydroxyl) atoms as well; there is also the added possibility forsaturation of olefinic species which may be present. The electrolyticdenitrogenation is carried out by the cathodic reduction of the nitrogenspecies to ammonia in a cell analogous to a proton exchange membrane(PEM) fuel cell. In the cell, a source of hydrogen is oxidized in theanode compartment to form protons which permeate through the membraneelectrode assembly (MEA) which divides the cell to the cathode sidethrough which the jet fuel is passed. The anode reaction can berepresented as:

H₂→2H⁺+2e⁻

The reduction of the nitrogen species in the jet fuel takes place in thereaction at the cathode side which may be represented simplistically as:

2R—N—H+6H⁺+6e⁻→2R—H+2NH₃

Proton Exchange Electrolytic Cell

A much simplified diagrammatic section of a proton exchange electrolyticcell suitable for carrying out the present denitrogenation reaction isshown in the FIGURE. The electrolytic cell 1 contains and anodecompartment 2 and a cathode compartment 3 separated by a membraneelectrode assembly composed of catalytically-active,electrically-conductive anode 4, a catalytically-activeelectrically-conductive cathode 6 which form the faces of a permeablemembrane 6 in the two respective compartments. Anode 4 and cathode 3 areconnected to a source of dc current 7 to provide power to the twoelectrodes during operation. The anode compartment is provided with aninlet 10 for the hydrogen source and an outlet 11 for excess notconsumed in the reaction. The cathode compartment is similarly fittedwith an inlet 12 for the fuel and an outlet 13 for fuel containing thedenitrogenated species. The anode and cathode compartments areconstructed to be narrow so that they each constitute a flow passage forthe respective process fluid (hydrogen source or jet fuel) whichoptimizes contact of the fluid with the respective electrode.

The hydrogen source may suitably be hydrogen gas or water. Otherhydrogen sources susceptible to electrolytic oxidation may also be used,for example, methanol. If hydrogen itself is used, the effluent from theanode compartment will be excess hydrogen which can be recycled to theinlet; it is not, however, necessary to maintain a high flow regimeusing hydrogen since essentially all the hydrogen is consumed in theanode reaction and transported to the cathode where the reductionreactions are effected. If water is used, the effluent will containoxygen formed in the cathodic oxidation which can be separated from thewater in a simple gas/liquid separator before recycling the water, ifdesired.

The anode and cathode of the membrane electrode assembly are separatedby a thin, proton-conducting membrane with the respective electrodematerials attached directly to its opposing surfaces. Suitable anodesinclude graphite, platinum, platinum-coated titanium, or ruthenium oxidetitanium oxide-coated titanium (the so-called dimensionally stable anodematerials). The electrolytic cathode is an electrically conductivecatalytic material having hydrogenation activity. Suitable materialscomprises a finely divided metal such as Raney-type metals (e.g.,nickel, cobalt, copper, molybdenum), Raney alloys (e.g.,nickel-molybdenum and nickel-cobalt), and high surface area precious(noble) metals (e.g., platinum black, ruthenium black, and palladiumblack as well as palladium-loaded carbon powder).

The cathode can have several configurations. The cathode can consist ofa finely divided catalyst powder layered in a bed about 100-300 micronsthick (although thicker beds have no deleterious effects on thehydrogenation reaction). The bed is prepared by allowing the catalystparticles to gravity-settle (coat) onto a flat sheet current collector.The particles in the bed must contact one another for the appliedcurrent to pass from one particle to another. The cathode can consist ofa mixture of catalyst particles and an inert binder such aspolytetrafluoroethylene (PTFE) rolled into a flat sheet and affixed tothe surface of the membrane.

The membrane comprises a proton exchange membrane which permitspermeation of the protons while inhibiting passage of the gases, liquidsand the electrons generated in the anode reaction. A proton exchangemembrane (PEM) is a semipermeable membrane generally made from ionomersand designed to conduct protons while being impermeable to gases such asoxygen or hydrogen. PEMs can be made from either pure polymer membranesor from composite membranes where other materials are embedded in apolymer matrix. One of the most common and commercially available PEMmaterials is the fluoropolymer (PFSA) Nafion® from DuPont. Nafion is anionomer with a perfluorinated backbone and pendant sulfonate groups inthe H⁺ form produced by incorporating perfluorovinyl ether moietiesterminated with sulfonate groups onto a tetrafluoroethylene backbone.Nafion polymers are noted for their excellent thermal and mechanicalstability, making them highly suitable for use in electrolysis cellsalthough their preferred range of operating temperatures is typicallyless than 100° C. and for best functioning, less than 80° C. e.g. 75 or60° C. Nafion polymers will tend to dehydrate (thus losing protonconductivity) when temperature is significantly above ˜80° C. butversions capable of operation at higher temperatures can be made byincorporating silica and zirconium phosphate into the polymers toincrease the working temperature to above 100° C. Nafion polymers can beextrusion cast into thin membranes, e.g. from about 125 to 250 microns,to form membrane assemblies. Other ionomers also exist, however, whichare useful for proton exchange membranes and which can be used at highertemperatures. Other ionomer materials with potential for highertemperature operation include sulfonated polyetheretherketones,sulfonated polysulfones, polybenzimidazoles, diaminobenzidines andpolybenzimidazole/poly(tetrafluoro ethylene) composites. The membraneexchange assembly is suitably made by connecting or depositing theelectrode material on both sides of the membrane. If elevated pressuresare contemplated, a structural reinforcement may be provided with theuse of a permeable support material which may be pressed onto themembrane.

The process is preferably carried out at normal atmospheric pressuresand a temperatures of about 25 to about 75° C., e.g., 60° C. Elevatedpressures are not required but may be used (e.g. 50-100 kPag) providedthe mechanical integrity of the MEA is preserved in the reactor. Mildlyelevated pressures may have a beneficial effect by helping to maintain ahigh hydrogen concentration on the cathode surface, promoting permeationthrough the membrane assemble to affect the hydrodenitrogenation.

EXAMPLE

A 500 ml sample of kerosene (API gravity=43°, boiling range=110-290° C.(about 228-550° F.), freeze pt=−46° C.) was circulated through thecathodic side of a polymer electrolyte membrane fuel cell. The fuel cellis a commercially available product from Scribner Associates, ofSouthern Pines, N.C., consisting of two carbon blocks with serpentineflow channels designed for a 5×5 cm active area of catalyst. Themembrane electrode assembly was purchased from Lynntech, Inc of CollegeStation, Tex. and consisted of a Nafion® 117 polymer electrolytemembrane with a 2.5 mg/cm² anode catalyst layer of platinum black and a2 mg/cm² cathode catalyst layer of palladium black. The catalyst layerswere first deposited on Toray Paper TGP-H-060 gas diffusion materialbefore being hot pressed to the Nafion. The kerosene was circulated at aflow rate of 50 cm³/min while simultaneously passing hydrogen gas atatmospheric pressure through the anodic side of the membrane electrodeassembly at a flow rate of 100 cm³/min. The cell temperature wasmaintained at 70-75° C. throughout the run of two hours. A constantcurrent of 1 amp was applied to the cell during the run. The nitrogencontent was found to be reduced from an initial concentration of 22 wppmdown to a 1 wppm level after two hours of treatment.

As a control experiment, the procedure was repeated identically, exceptno power was applied to the fuel cell. In this instance, the nitrogenlevel dropped to 13 wppm, which we attribute to adsorption of thesurface active nitrogen species on the carbon support of the MEA.

The storage stability of the untreated and electrochemically treatedkerosenes were tested by storing two 100 ml samples of each in closedglass containers, in the dark, at ambient temperatures for two years. Atthe end of this test, the untreated kerosene had darkened to a yelloworange color and contained solid matter. The treated kerosene remained astraw color and did not have any sediment visible to the eye. Toquantify the solids, both solutions were filtered through Whatman™ 47 mmdiameter glass microfiber filters. The filters were dried in a vacuumoven at 100° C. until constant weight was achieved. The untreatedkerosene produced the equivalent of 500 mg/L of dry solids, whereas thetreated kerosene only produced 40 mg/L. To compare the discoloration,visible light absorbance at 400 nm was measured for each. The untreatedkerosene had an absorbance of 0.40 absorbance units, whereas the treatedsample had an absorbance of 0.11 absorbance units.

Close inspection of the fuel by Electrospray Ionization (ESI) MassSpectrometry (KQ and AM) provided further insight into the types ofnitrogen species present in the fuel sample and the changes incomposition that occurred during the control experiment without powerand the proof-of-principle experiment with power. It was found that notonly are nitrogen species removed, but also mixed heterocyclic speciescontaining one nitrogen (pyridinic, quinolinic) with one sulfur(thiophenic) or oxygen (hydroxyl) atoms.

1. A method for the denitrogenation of a distillate boiling range jetfuel which comprises cathodically hydrogenating nitrogenous componentsof the fuel in a cathode compartment of a divided electrolytic cellhaving a proton permeable membrane separating the cathode compartmentfrom an anode compartment in which a source of hydrogen is anodicallyoxidized to form protons.
 2. A method according to claim 1 in which theproton permeable membrane comprises a membrane electrode assemblycomprising a having a catalytic anode surface and a catalytic cathodesurface on opposing surfaces of the membrane.
 3. A method according toclaim 1 in which the proton permeable membrane comprises an ionomer. 4.A method according to claim 1 in which the proton permeable membranecomprises sulfonated poly(tetrafluoroethylene).
 5. A method according toclaim 2 in which the catalytic anode surface comprises a noble metal. 6.A method according to claim 5 in which the catalytic anode surfacecomprises platinum or palladium.
 7. A method according to claim 2 inwhich the catalytic cathode surface comprises a an electricallyconductive catalytic material having hydrogenation activity.
 8. A methodaccording to claim 7 in which the catalytic cathode surface comprises afinely divided Raney-type metal.
 9. A method according to claim 7 inwhich the catalytic cathode surface comprises a noble metal.
 10. Amethod according to claim 7 in which the catalytic cathode surfacecomprises platinum or palladium.
 11. A method according to claim 1 whichis carried out at a temperature of not more than 80° C.
 12. A methodaccording to claim 1 in which the distillate boiling range jet fuelcomprises a kerosene having an initial boiling point of not less than150° C. and an endpoint not more than 300° C. (ASTM D86).
 13. A methodaccording to claim 1 in which nitrogenous components of the fuel arehydrogenated to form ammonia which is released from the treated fuel ina liquid/gas separator.
 14. A method according to claim 13 in theammonia is released from the treated fuel by stripping with inert gas.15. A method according to claim 13 in the ammonia is released from thetreated fuel by increasing the temperature of the treated fuel.
 16. Amethod according to claim 1 in which the hydrogen source compriseshydrogen gas.
 17. A method according to claim 1 in which the hydrogensource comprises water.